A solid polymer electrolyte fuel cell has a structure comprising a solid polymer electrolyte membrane as an electrolyte and catalyst electrode layers bonded to both sides of this membrane. Such solid electrolyte membrane and catalyst electrode layers composing the solid polymer electrolyte fuel cell are generally formed using polymer electrolyte materials having proton conductivity. As such electrolyte materials, perfluorosulfonic acid-based resins such as Nafion (trademark: DuPont) have been widely used.
Meanwhile, fluoride-based and hydrocarbon-based electrolyte membranes that are used for solid polymer fuel cells become thinner when electrolyte polymers deteriorate due to OH radicals generated upon electric power generation. A means for suppressing such deterioration involves adding a radical scavenger such as CeO2 to a catalyst layer or a diffusion layer of a membrane electrode assembly (MEA), so as to improve the resistance of MEA to radicals. In this case, the radical scavenger added to the catalyst layer or the diffusion layer migrates into the membrane with time.
For example, the following JP Patent Publication (Kokai) No. 2007-213851 A discloses that, for the purpose of improving the durability of a polymer electrolyte membrane, a hydrogen peroxide decomposing agent, a radical scavenger, or an antioxidant is preferably contained in a diffusion layer.
Aside from this, there are methods that involve directly adding a radical scavenger to an electrolyte membrane. Methods for directly adding a radical scavenger to an electrolyte membrane are broadly classified into methods that involve adding it after membrane production via an ion exchange method or the like and methods that involve mixing an electrolyte polymer with a radical scavenger before membrane production. Particularly when a solid such as an oxide is added, the latter methods are desirable. Furthermore, when membrane production is carried out by casting, a radical scavenger is added to and mixed with a cast liquid in advance for casting. When membrane production is carried out by melt-molding, an —SO2F type polymer, which has a precursor structure of an electrolyte, is mixed with a radical scavenger and then hydrolysis is carried out after melting and membrane production, thereby converting the polymer to an —SO3H type polymer that can undergo proton conduction.
In the case of a membrane production method via melting, membrane hydrolysis is carried out after addition of a scavenger. In the case of a radical scavenger such as CeO2 or Ag2O, which is relatively easily dissolved in acid, there is a problem that during acid treatment, partial or the entire volume of such a radical scavenger in the membrane is dissolved and then flows out from the membrane, so that the radical scavenger that should already be added upon completion of the electrolyte membrane is lost during membrane preparation. As a result, a problem that sufficient resistance to radicals cannot be imparted to the membrane also occurs.
A radical scavenger is thought to deactivate radicals by itself or in the form of cation thereof generated as a result of dissolution thereof. Dissolution of a radical scavenger itself is not a problem. Actually, the interior of an electrolyte membrane is acidic atmosphere. It is thought that during operation of a cell, acidic water is also present within the membrane because of external humidifying water or generated water and a radical scavenger is eluted thereinto to generate cations and thus to deactivate radicals. A problem is that a radical scavenger expected to be eluted and act during operation of a cell is eluted during membrane preparation because of its exposure to acid, so that the scavenger is partially or completely lost. Therefore, it has been an issue to find a method that involves suppressing elution of a radical scavenger due to acid in a hydrolysis step, so as to enable sufficient exertion of the functions of the radical scavenger with no problems during operation of a cell, although the radical scavenger in the membrane is exposed to acidic atmosphere in both the membrane hydrolysis step and the operation of a cell.